If I understand you correctly, you're double-clicking on the clip in a timeline and having it open in the Source Monitor, yes?
That behavior is correct by default, because you're opening up a clip with set boundaries (an In and Out point) which affect the clips on either side of it. What you're seeing is the full length of the clip based on those In and Out points, with the zoom slider set accordingly; if it was shorter clip the zoom slider would be wider.
If you want to reveal the original source clip and not worry about these restrictions then do a Match Frame (F) instead of double clicking.